This invention relates to the shrinkproofing of wool sliver. Disclosed herein is a novel process and an apparatus particularly suitable for carrying out the novel process. Also disclosed herein is a particular solution which is advantageous in the shrinkproofing of wool, not only with the novel apparatus and process disclosed herein but generally in the field of shrinkproofing of wool.
Wool sliver is a commercial product which is produced in a variety of relatively uniform sizes. Sliver is normally described in terms of its quality and in terms of the weight of a 5 yard length of material. Thus, a 21/2 ounce sliver indicates that this particular sliver would weigh 21/2 ounces for a 5 yard length. This sliver would be approximately 1 inch in diameter.
In the normal processing of wool, wool fibers are obtained by clipping from an animal. The clipped fibers are then baled. After collection in bales the raw wool may be scoured and carded. In this state the wool is referred to as card sliver. Card sliver upon combing to remove adventitious materials receives a slight twist and is referred to as top sliver. Top sliver may be drawn such that its diameter is reduced at which time it is then referred to as roving. Roving may be twisted into yarn. Several plies of yarn may be then twisted to form a thread. Generally speaking, wool will be treated while in either the card sliver or top sliver state. However, the invention disclosed hereinafter is useful in any of the stages from card sliver through to thread. Accordingly, the word "sliver" as used hereinafter is intended to be broad enough to encompass card sliver, top sliver, roving, yarn and thread.
It is well known in the prior art that wool sliver may be shrinkproofed by immersing it in an aqueous hypochlorite solution. There has been much technical study and many patents in this field.
It is generally acknowledged that the major reactant involved is hypochlorous acid which is one of the entities in wet chlorine systems, and that the reaction should, as far as possible, be confined only to the surface of the fibers. The reaction between wool and hypochlorous acid tends to be exceedingly rapid and difficult to control so that the major difficulty is one of obtaining "even treatment" of the fibers.
Since the chemical equilibrium involved in HOCl solutions is very pH dependent and since the by-product of the HOCl/wool reaction is the completely dissociated HCl, a self accelerating action is set up at the original reaction site.
Since the rate of the wool/hypochlorous reaction is so fast, it is possible for some of the fibers to be "wetted" by reactant solution which has lost its HOCl content in a first reaction. It is obvious, then, that if the reactant solution is to maintain homogeneity, the rate of its dispersion through the fibers must be faster than the wool/hypochlorous reaction rate if the desired end of even treatment is to be met. To achieve this, several methods have been used to reduce the reaction rate including temperature control; pH modification and addition of sacrificial amino compounds. Any or all of these methods have been coupled with agitation, vibration or wetting agent additions to speed the rate of liquid dispersion. Despite all of this work, the attainment of even treatment is still a major difficulty.
It is in the nature of this invention, that even and homogenous treatment of combed wool sliver can be obtained by use of a mechanical procedure designed to take specific advantage of the geometry of combed wool sliver.
The invention is based in the discovery that the rate of air displacement by the solution for sliver continuously immersed within 10.degree. of the vertical is smooth and even, whereas for sliver immersed horizontally the rate is slow and discontinuous. Because the sliver consists of combed wool fibers, air trapped in the capillaries will be smoothly and uniformly displaced only if the sliver is introduced in the bath in a substantially vertical direction. Immersion of the sliver in the bath in a horizontal direction or nearly horizontal direction will result in entrapment of the air in the capillaries of the sliver and thereby resulting in uneven shrinkproofing of the wool.
Heretofore, wool sliver has been immersed in an appropriate bath at any convenient angle which normally will approach the horizontal. The sliver then travels in a more or less horizontal plane through the bath for an appropriate amount of time to permit treatment by the solution. In order to assist in the treating of the sliver with this type of bath and immersion arrangement the bath is often agitated.
According to this invention the wool sliver is introduced into the bath in a substantially vertical direction and travels downward through the solution in a substantially vertical direction to an appropriate depth. By a suitable choice of the depth to which the sliver is immersed and the rate at which it is carried into the bath, it is possible to obtain even and homogenous filling of the capillary spaces between the combed fibers with reactant liquid in a time less than that required for the wool/hypochlorite reaction. The combed wool sliver (whose capillaries are filled with air) is conveyed substantially vertically into the reactive chlorine solution to a depth of not less than 1 meter at a rate of about 7 centimeters to about 25 centimeters per second. The sliver must be submerged substantially vertically to a depth of at least 1 meter in order that all air may be removed from the capillaries in order that even treatment of the sliver will be achieved. Because the sliver is a relatively delicate product which cannot be subjected to tensile stresses of any significance the sliver may most advantageously be carried into the bath between or on endless screens.
While the sliver is being submerged the air is smoothly displaced from the capillaries by the chlorine liquor. The screens carry the combined wool sliver and capillary liquor around a roll or guide at the bottom of the tank and up through the solution and out of the bath exit.
The sliver now evenly wetted with reactant liquor, is allowed to complete its reaction and is then squeezed, rinsed free from spent liquor and dried.
Sliver so treated is very evenly shrinkproofed and the secondary procedures of attempting to control the rate of reaction by adjustment of pH, of temperature or by adding amino compounds; or of improving the wetting rate by the use of surfactants or agitators or vibrators seem to be unnecessary. The degree of shrinkproofing is easily controlled by adjusting the concentration of hypochlorous acid in the solution.
In essence, an apparatus designed according to this invention consists of a bath of sufficient depth to permit immersion of sliver to a depth of at least 1 meter of fluid. The apparatus will consist of a pulley or guide at the top over which the sliver and its conveying means may be made to pass. A similar pulley or guide will be located at the bottom of the tank. And thirdly, a similar pulley or guide will be located at the exit to withdraw the sliver and conveying means from the tank.
Normally, the final guiding member will be located atop the bath in such a way as to withdraw the wool sliver from the bath as quickly as possible. As the feed rate of sliver is governed by the immersion rate into the bath, obviously withdrawal from the bath in minimum time may only be achieved by removing the sliver according to the shortest path, i.e. vertical. This is desirable although not necessary in order to prevent secondary oxidation by diffusion of more hypochlorous acid. If the sliver is overtreated wool of an unsatisfactory quality and color will result. However, if suitable sealing means could be provided it would be permissible within the terms of this invention to remove the wool sliver from the solution in any particular direction.
Over-treatment of the wool may be easily prevented by use of a U-shaped bath. If a suitable hypochlorous solution is introduced into such a bath at a point relatively near to the point where the wool sliver is introduced, the liquor is fresh. By ensuring a flow of liquor along the U-shaped bath in the same direction of flow as followed by the wool the liquor relatively near the point where the wool is withdrawn from the bath will be weak or spent. Thus, the liquor is caused to flow down one side of the U-shaped bath in which the sliver is traveling downwardly and up in the other leg of the bath. Supply means are incorporated in upper portions of the first mentioned leg of the bath and drain means are provided at the upper end of the second mentioned leg. As the wool sliver is removed from the bath it will of course remove with it some entrapped liquid.
The solution to be used in such a bath is a hypochlorite solution. The best solution known to the inventor is made by passing chlorine gas into water. Such a solution contains less chloride ion than solutions commercially used today, and accordingly has a higher concentration of hypochlorous acid per unit of available chlorine.
Aqueous solutions of hypochlorite are characterized by the uneasy equilibrium between the components. The equilibrium is very pH dependent and can be characterized thus: EQU OCl+H.sup.+ .revreaction.HOCl+H.sup.+ +Cl.sup.- .revreaction.H.sub.2 O+Cl.sub.2 14.THETA.pH.fwdarw.1
Consequently, the effective concentration of the HOCl entity is dependent on both the total available chlorine of the solution and its pH value.
Shrinkproofing procedures usually operate with solution of about 0.05% hypochlorous acid obtained by different combination of pH and total chlorine (Harris, U.S. Pat. No. 2,466,695; Edwards, British Pat. No. 537,671; Kroy, U.S. Pat. No. 2,671,006). In summary, the overall reaction could be designated as: EQU Wool+HOCl.fwdarw.oxidized wool+H.sup.+ +Cl.sup.-
The by-product of this reaction (the hydrogen and chloride ions) when released have a very important effect upon the hypochlorite/hypochlorous equilibrium.
In the upper pH conditions (pH 5 to pH 14) the released hydrogen ion converts the unreacted hypochlorite ion to the relatively undissociated hypochlorous acid: EQU OCl+H.sup.+ .fwdarw.HOCl
This secondary hypochlorous acid can produce localized secondary oxidation which may continue so long as hypochlorite ion is available.
Consequently, alkaline hypochlorination (so-called) tends to produce more secondary oxidation (damage) per degree of primary oxidation (shrinkproofing) and this shows as increased damage to the wool fibers.
To offset this, according to prior art procedures, recourse is had to a very rapid treatment with the hypochlorite solution followed by immersion in a stop bath to destroy the residual hypochlorite. The relatively slow rates of liquid diffusion through wool fibers and the exceedingly fast rate of the wool hypochlorite reactions and hypochlorite/hypochlorous equilibria make the procedure exceedingly difficult.
In acid conditions (pH 5 or less) the release of by-product HCl decreases the HOCl concentration: EQU HOCl+H.sup.+ +Cl.sup.- .revreaction.H.sub.2 O+Cl.sub.2
and it is interesting to note that since the by-product releases of H.sup.+ and Cl.sup.- are stoichimetric the rate of the reaction is proportional to the square of the hydrogen ion release concentration. ##EQU1##
Hence release of by-product H.sup.+ and Cl.sup.- from the initial shrinkproofing reaction produces a very dramatic fall in pH equilibrium HOCl content of the shrinkproofing solution.
Consequently, in acid solutions the oxidation by hypochlorous acid is self-limiting at the primary stage and the result is shown in the much lesser degree of damage per effective degree of shrinkproofing.
Commercial hypochlorite solutions have available chlorine values of 1% to 16%. They are invariable alkaline (generally pH 12). They are formed by passing gaseous or liquid chlorine into cooled alkaline solutions. The reaction can be represented thus: EQU Cl.sub.2 +2NaOH.fwdarw.NaCl=NaOCl+H.sub.2 O+heat
The reaction is generally stopped a little short of the stoichimetric point so as to keep the solution at pH 12.
To make hypochlorous acid solution such commercial hypochlorite solutions are progressively diluted and neutralized with mineral acid, generally muriatic acid.
There are several objections to this procedure:
1. Alkaline hypochlorite solutions undergo steady degradation with time and with temperature thus: EQU NaClO.fwdarw.Na.sup.+ +Cl.sup.- +(O).uparw. PA1 Consequently as they age the concentration of chloride ion as a function of available chlorine steadily increases. PA1 2. Acidification means not only the conversion of NaClO to HClO but also the neutralization of the excess alkali in the solution. Consequently this, too, produces an increased concentration of chloride ion as a function of the total available chlorine. PA1 3. The heat of neutralization effectively increases the temperature of the hypochlorous acid solutions and since hypochlorous acid is quite volatile, cooling is required to produce satisfactory shrinkproofing solutions. In such acidified solutions the equilibrium is expressed as: ##EQU2##
It is obvious that increases in the chloride ion concentration of the solution produce a corresponding reduction in the true [HOCl] per unit of total available chlorine.
Contrary to commercial practice today, the pH and available chlorine values are not sufficient to determine the true concentration of HOCl. To determine true [HOCl] consideration must be given to the chloride ion concentration.
Consequently, acidified solutions of commercial alkaline hypochlorite solutions are lower than expected in true concentration of hypochlorous acid.
However, fresh hypochlorous acid solution can be readily obtained by passing chlorine gas into water: EQU Cl.sub.2 +H.sub.2 O.fwdarw.H.sup.+ +Cl.sup.- +HClO
Such a solution proves to be more advantageous for shrinkproofing because it contains less chloride ion (than the previously described acidified commercial product) and therefore has a higher concentration of hypochlorous acid per unit of available chlorine.
It will be appreciated from the previous description that a solution prepared according to the foregoing method will be essentially self limiting in the wool shrinkproofing reaction. Accordingly, it may be used in any form of bath wherein the solution is evenly distributed throughout the wool sliver. Since the reaction is self limiting, it is not necessary to subject the wool sliver to after bath treatment to stop the chemical reaction in order to obtain acceptable quality shrinkproof wool.
In accordance with this invention, effective shrinkproofing solutions may be obtained by aspirating chlorine into the feed water system of the shrinkproofing process. Solutions of available chlorine (0.05 to 0.25) at pH values at 2.0 to 2.6 and temperatures of 6.degree. C. to 20.degree. C. are easily obtained. As a consequence, neither artificial cooling nor cumbersome neutralization procedures are required according to the invention disclosed herein. It will be particularly obvious that the solution disclosed herein may be used advantageously in conjunction with the method and apparatus disclosed herein to provide a single bath treatment of wool wherein the wool is uniformly treated with little or no danger of secondary treatment and damage to the wool.